Block copolymers of ethylene oxide and propylene oxide are well known nonionic surfactants. Their unique hydrophilic/hydrophobic character translates into a broad range of surfactant functions and physical properties. Their exceptional versatility has led to their widespread use in diverse markets and applications. Many of these surfactants are valued for their defoaming/antifoaming, cleaning, solubilization, foaming, emulsification and wetting properties, among others.
EO/PO block copolymer surfactants also function extremely well as dispersants in aqueous media, particularly in pesticide formulations. A dispersion is a system containing solid particles in a liquid continuous phase. A pesticidal dispersion will therefore contain particles of active material spread throughout the liquid medium. A good surfactant or dispersant will therefore inhibit the natural tendency of the solid particles to stick together, or recombine. Flocculation and hard agglomeration are thus two problems which dispersants are designed to help prevent.
The dissolution rates of the EO/PO block copolymers will to a large extent determine their suitability as dispersants of active solids. The better the dissolution rate, the better the ability of the surfactant to stabilize the solid in the liquid system. Unfortunately for many EO/PO block copolymers, their dissolution rates can be sufficiently slow so as to preclude their use as dispersants in many instances.
One particularly efficacious group of nonionic surfactant dispersants is represented by the formula: ##STR1## where x, y and x' are numbers such that the block copolymer is greater than 50% hydrophilic and often more than 70% hydrophilic, and the molecular weight (M.sub.w) of the hydrophobe is between 950 and 4000. These compounds have total molecular weights in excess of 1000 and are marketed under the trademark PLURONIC.RTM. by BASF Corporation. Of special note is the EO/PO block copolymer represented by the above formula and having a total molecular weight of about 8,400. This compound is available under the trademark PLURONIC.RTM. F68.
While the above compounds have been used for many years as dispersants, notably as additives in pesticide formulations, there has been a need to improve the dissolution rates of these compounds without sacrificing their performance characteristics such as suspensibility and settling. Currently, the dissolution rates for the compounds described above are in excess of 11 minutes. While this has been satisfactory for end users of pesticide formulations, it is not ideal.
Anionic surfactants have been combined with nonionic surfactants such as EO/PO block copolymers. Holland et al., U.S. Pat. No. 5,169,894 relates to dispersants for inorganic pigment slurries with polycarboxylates and nonionic EO/PO copolymer surfactants. In this context, the nonionic surfactant is utilized to lower anionic loading on the pigment, and the polyacrylate is the major component of the blend in amounts of as much as 90%. Holland et al. fail to discuss how to increase the dissolution rate of the nonionic surfactant through addition of the anionic component thereto.
Thus there presently exists a need in the art for one or more dispersants made from EO/PO block copolymer surfactant blends having considerably faster dissolution rates and excellent performance features.